As the pre-treatment of metal for electrocoating, there has heretofor been adopted phosphating, which has been carried out by either one of spraying, dipping or combination of dipping and spraying means. The spray process is advantageous in that it can save the installation cost and improve the production efficiently. However, in case of articles of complicated shapes which have many pocket portions, there are problems such that there are areas to which direct spray of a phosphating solution is not feasible and areas with only poor qualities due to splashes of the phosphating solution. Whereas, the dip process is, though the installation cost is rather high, much preferable to spray process for the articles of complicated shapes, since it is able to form a uniform film.
However, in the heretofor proposed dip treatments, it is generally recognized that in order to get a phosphating film, said treatment must be carried out with a phosphating solution containing a high concentration of zinc ion (2 to 4 g/l) at a high temperature (60.degree. to 90.degree. C.) for a long period of time (3 to 10 minutes). The formed film has a large film weight (3 to 5 g/m.sup.2) and because of poor adhesion, low corrosion resistance and inferior appearance, is not suitable as a base for electrocoating. In recent years, electrocoating compositions to be used in an automobile industry have been changing from of anion type to of cation type so as to assure a satisfactory rust-proof effect even under various environmental conditions. Different from anionic electrocoating compositions, cationic electrocoating compositions form a coating film as the result of liberation of an alcohol blocking the crosslinking agent therein on baking, and therefore, the coating film is greatly shrinked and a considerable force acts on the phosphate film provided thereunder. Thus, the phosphate film as a base for cationic electrocoating is required to have a sufficient strength tolerable to the said shrinkage.
Under the circumstances, Nippon Paint Co., Ltd of Osaka, Japan recently filed a patent application, Japanese Patent publication (unexamined) No. 107784/1980, on a phosphating method of treating iron-based metal surfaces which is particularly suitable for treating manufactured products having complicated surfaces, such as automobile bodies.
The above phosphating method is in use commercially in the automotive industry for pretreating automobile bodies prior to cationic electrocoating. This method is carried out by first subjecting the metal surface to a dipping treatment with an aqueous acidic solution containing 0.5 to 1.5 g/l of zinc ion, 5 to 30g/l of phosphate ion, and 0.01 to 0.2 g/l of nitrite ion at a bath temperature of 40.degree. to 70.degree. C. for 15 to 120 seconds, followed by spraying with the above solution for 2.about.60 seconds for sludge removing purpose, and is reported to be capable of providing a phosphate film of relatively low film weight (1.5 to 3 g/m.sup.2) which is effective for forming a coating by cationic electrocoating having excellent adhesion and corrosion-resistance on complicated articles.
Recently, in the automotive industry, consistent with the aim of further improving corrosion-resistance after the application of a siccative coating, steel components which are plated on one surface with zinc or a zinc alloy have come to be used as materials for automobile bodies. When the process of the above Japanese Patent publication is applied to such materials (i.e. to metal components having both iron-based metal surfaces and zinc-based metal surfaces), the iron-based surfaces are provided with a phosphate coating film having a low film thickness with uniform and dense cube crystals, as well as excellent adhesion and corrosion-resistance. Such phosphate coating on the iron-based surface is suitable as a substrate for cationic electrocoating. However, in the case of the phosphate coating film formed on the zinc-based surfaces, the resistance to salt spraying after the application of a cationic electrocoat thereto is insufficient, and secondary adhesion (by immersion test of the film with cross-hatched scratches in warm water) after cationic electrocoating--intermediate intermediate coating--top coating is greatly inferior to that on the iron-based surfaces.
To cope with the same, were provide, in Japanese Patent publication ((unexamined) No. 152472/1982), a technique of using an aqueous acidic phosphating solution comprising from 0.5 to 1.5 g/l of zinc ion, from 5 to 30 g/l of phosphate ion, from 0.6 to 3 g/l of manganese ion, and/or 0.1 to 4 g/l of nickel ion and a phosphating accelerator, and in Japanese Patent publication No. 36588/1986, a technique of using the combination of manganese ion and a fluoride ion in a phosphating solution.
By these methods, a phosphate coating film which is suitable for cationic electrocoating can be formed on iron-based metal surfaces, zinc-based metal surfaces or combination of these surfaces by dip treatment with an aqueous acidic phosphating solution and such dip treatment has acquired a firm, advantageous position in the phosphating processes for the purpose of improving corrosion-resistance of various kinds of metals including iron, zinc and alloy metals, for automobile bodies and parts, building materials and other small articles. Recently, with the increasing demand for quality cars, a far better anti-corrosive nature is longed for on the phosphate coating film. The film should preferably be well resistive toward hot brine dipping test and scab corrosion test. Unfortunately, the heretofor proposed phosphating processes have failed to meet the present quality requirements.
On the other hand, in the case of steel furnitures or the similar products, the spray process is still in the main current. However, even in that field, galvanized steel is getting increased in consumption and improvements in adhesion and corrosion-resistance, and especially scab corrosion resistance and hot brine dipping resistance are highly desired, it is an object of the present invention to provide a process for phosphating metal surfaces including iron-based surfaces, zinc-based surfaces and combination of these surfaces, resulting a phosphate film capable of providing excellent adhesion and corrosion-resistance to coatings from electrocoating and especially from cationic electrocoating.
A further object of the invention is to provide a process for phosphating metal surfaces, whereby the scab resistance of iron-based surface and hot brine dipping resistance of both iron-based and zinc-based surfaces after the application of a cationic electrocoat thereonto are greatly improved and secondary adhesion after cationic electrocoating, intermediate coating and top coating is likewise further improved.
A further object of the invention is to provide an aqueous acidic zinc-phosphating solution to be used in the present phosphating process.
An additional object of the invention is to provide an aqueous concentrated composition for formulating said aqueous acidic phosphating solution. An additional object of the invention is to provide phosphated metal surfaces obtained by the process of this invention. Other objects and advantages of the present invention will become apparent from the following disclosure.
According to the invention, the abovementioned objects can be attained with a process for treating a metal surface with an aqueous acidic zinc-phosphating solution comprising about from 0.01 to about 200 g/1 as tungsten of soluble tungsten compound, and preferably, an aqueous acidic zinc-phosphating solution containing as essential components, from about 0.1 to about 2.0 g/l of zinc ion, from about 5 to about 40 g/l of phosphate ion, from about 0.01 to about 20.0 g/l as tungsten of soluble tungsten compound and a conversion coating accelerator.
The metal surfaces treated in accordance with the present invention include iron-based surfaces, zinc-based surfaces and combination of these surfaces. The term "treatment" as used in the present invention shall mean dipping, spraying or combination thereof. However, since there are miner variations in the details of such treatments and compositions of aqueous acidic zinc-phosphating solution used, the invention shall be now more fully explained separately for each treatment.